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Re: Preamp bypass circuit



> From:          "Duffin, Jon" <jduffin@dbxpro.com>
> To:            "'amsat-bb@amsat.org'" <amsat-bb@AMSAT.Org>
> Subject:       [amsat-bb] Preamp bypass circuit
> Date:          Wed, 14 Feb 2001 12:15:22 -0700

> I have recently built up a GasFET preamp for use on the 2 meter band on
> AO-10.  To switch the preamp out during transmit, I have built up a RF
> sensed circuit which bypasses the preamp through a relay.  The problem is
> that I can observe a breif RF transient that would go into the preamp (I
> actually have not hooked up the preamp to the switch circuitry yet) before
> the relay switches into the bypass mode.  I am fearful that this RF
> transient would be enough to blow out the GasFET.  Does anybody have any
> hints or ideas how to supress this transient so I won't be replacing the
> FET?
> 
> My rig is a FT847, and so I though another idea would be to switch the
> preamp out of the transmit path by using the 847's PTT accessory jack.  But,
> I don't think that the results would be any different unless there is a way
> to delay the power output stage from coming up at the same time as the
> switching of the PTT accessory jack signal.
> 
> How does everyone else get this to work on their homebrew preamps?
> 
> Thanks
> Jon
> KK7RZ

Dear Jon,

What you need is called a squencer.  There has been a schematic of one
in every issue of the ARRL Handbook for years.  Trust me, they are simple
to build and will save you a world of problems.  You can buy the PC board
from FAR circuits.

A squencer is simply a staged time delay circuit.  Instead of keying your 
847 directly, you key the squencer.  First it keys the relay(s) in your pre-amp 
to the transmit position.  Then after a very brief delay, it keys your transmitter.
When you let up on the mike PTT, it unkeys the radio first, then switches
the pre-amp relays.  Simple!  

This is a one IC circuit so the construction is simple.  Yes, you will have
to go inside your 847 and reroute the PTT connection from the mic jack
to the rear apron of the radio to a connector and then route it back in
to the PTT circuit again.  You can also steal the power for the squencer
and the ground connection through the same connector.

Please don't worry about your radios resale value.  Trust me, the addition
of a squencer connector will enhance it.  At least the next owner will know
that you were a serious VHF operator :-)!

You can also use the squencer to hard key your PA as well.  They work
much better when they are hard keyed instead of the RF sense circuit
(a really bad idea).  No more chattering relays!

BTW, I have one on both of my HF radios and my VHF and UHF multimodes.

Questions?  Let me know.

--73-- David WA0AUQ 
> 
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